Increasing demand for more powerful and convenient data and information communication has spawned a number of advancements in communications technologies, particularly in wireless communication technologies. A number of technologies have been developed to provide the convenience of wireless communication in a variety of applications, in various locations. This proliferation of wireless communication has given rise to a number of manufacturing and operational considerations.
A number of existing and emerging wireless communications technologies utilize a modulation scheme, known as Orthogonal Frequency Division Modulation (OFDM), to organize or allocate data transmissions across wide transmission bandwidths. Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (text, voice, video, etc.). OFDM extends the FDM scheme to a spread spectrum technique that distributes data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the “orthogonality”—which prevents demodulators from seeing frequencies other than their own. OFDM systems generally offer good spectral efficiency, resiliency to RF interference, and lower multi-path distortion. OFDM schemes are utilized in a number of common wideband and narrow-bandwidth communications systems and protocols—such as wireless LAN (WLAN) technologies based upon standards such as IEEE 802.11(a) & (g).
Within systems utilizing such protocols and technologies, a number of issues can arise in the detection of arriving packets, as well as from false triggers due to undesired packets or interference. For example, the 802.11(g) spectrum has 20 MHz wide channels, separated from one another by 5 MHz. In a number of applications, the presence of adjacent, potentially interfering channels at 5, 10, 15, and 20 MHz, etc., occurs often and must be addressed. As competing technologies and user deployments increase, such a problem is bound to become more acute.
Commonly, packet detection algorithms for OFDM WLAN packets exploit the fact that a short sequences part of a transmission preamble is periodic with period 0.8 μs—often establishing some type of elaborate timing sequence or scheme. Such approaches have some effectiveness, even in the presence of heavy multipath transmissions. Nonetheless, they are susceptible to frequent failures—particularly where they fail to discriminate between an inband signal (corresponding to a receiver's chosen channel) and a signal from an “overlapping” adjacent channel, since in both cases the short sequences are periodic with a period of 0.8 μs. Also, since the adjacent channel and inband channel signals overlap in frequency, mere filtering cannot alleviate the problem of false triggering on adjacent channel packets.
This false triggering (or detection) phenomenon gives rise to a number of systems performance or reliability problems. One such problem concerns an inability to detect inband packets. There may be instances where an initial false trigger on an adjacent channel packet causes the loss of any inband signal that may come in when this adjacent channel packet is being processed. This leads to a loss of data throughput. Other considerations involve throughput losses due to an inability to transmit. Where false triggers occur on adjacent channels too often, transmission could be blocked when needed—reducing throughput. Furthermore, if a false trigger occurs (on an adjacent channel) at the end of a packet, a CCA (clear channel assessment)—a signal that indicates to a media access controller (MAC) whether a channel is clear to transmit or not—may be lowered, preventing a MAC from transmitting an acknowledgment signal (ACK).
As a result, there is a need for a system that detects packet arrival of packets while, to the greatest extent possible, avoiding interference in a given wireless band—one that obviates performance irregularities by avoiding false triggers due to undesired packets or interference—while providing reliable wireless communication and data transfer in an easy, efficient and cost-effective manner.